In the past the usual method of attempting to measure the resistivity or conductivity of the earth's surface has been through the use of electrodes driven in the ground, applying a potential across such electodes and then measuring the current flowing between the electrodes through the ground. Such a method requires the utilization of survey lines and the establishing of good electrical contact between the electrodes and the ground with the result that the method is not only slow and cumbersome and provides a paucity of information but cannot be used at all where conditions such as winter or rocky terrain preclude the electrodes from establishing proper electrical contact with the ground to be measured.
Use has also been made of electromagnetic fields emanating from an infinitely distance source, for instance, those propagated from high powered transmitting stations and components of such fields have been measured through the use of coupled conductive stakes driven into the ground in alignment with the direction of propagation of the wave emanating from the station and a coil oriented with its axis perpendicular to such line to determine the apparent conductivity of the earth. The limitations on such methods are readily apparent when it is considered that the control of the emitted signal including its frequency is out of the hands of the geophysical surveyor and such methods provide limited conductivity information and are effective to very limited depths.
Still again, it has been proposed to measure the tilt of a created electromagnetic field to determine the apparent conductivity of the earth's surface but such methods have limited depth penetration and relatively limited information on conductivity distribution.
Again, use has been made of a large horizontal loop laid out on the ground as a transmitting coil to create an electromagnetic field and the magnetic field component out of phase with the transmitted field has been measured at a point remote for transmitter coil as a means for detecting vertical conductor bodies. Such a method has little utility insofar as measuring the conductivity of horizontal ground layers and has a very serious physical handicap in that it requires the trailing of a cable from the transmitting point to the receiving point to provide the reference signal in phase with the transmitter coil against which the phase shift in the detected magnetic field must be compared.